The Ministry of Defence abandoned plans for a secret computer database of UFO sightings for fear they would be mocked if its existence was ever revealed to the public, once-classified documents reveal.

The idea was scrapped because officials did not want people to believe they were taking the idea of visitors from outer space seriously. Instead, the MoD continued to keep only paper records of almost 1,200 sightings reported between 1987 and 1993. These include "pie shaped" flying saucers and diamond-shaped shards of light in the night sky. The files, to be posted on the National Archives website, include the story of an alien banana-man spotted in west London in May 1989.

Another mystery was the story of a woman who said that while walking her dog near Norwich in November 1989 she was approached by a man with a "flying suit" and "a Scandinavian-type accent". The report notes: "He asked her if she was aware of stories about large circular flattened areas appearing in fields of wheat, and then went on to explain that he was from another planet similar to Earth, and that the circles had been caused by others like him." The report said he disappeared and, as she ran away in fear, the woman saw a large glowing orange-white ball rising vertically from behind trees.

Other recorded incidents include a spate of UFO sightings in London in 1993 which were all put down to the presence of a brightly illuminated airship advertising the new Ford Mondeo, strange circular lights lighting up the sky in Rochdale in 1987, and various reports of strange objects in Derby in the same year, one of which "left a trail of bright blobs behind it".

An "elongated spinning top" apparently three times the size of an airship was spotted by a taxi driver in Huddersfield in 1990. The night before, he had reported a silver space craft emitting green, red and white beams. The MoD also looked into a photo of a diamond-shaped light hovering above an air base in Pitlochry. The bright light was spotted jerking vertically in a strange movement in August 1990.

Several documents cover the death of an US pilot attached to the RAF who crashed into the North Sea in 1970. Captain William Schaffner's death was investigated 20 years later after claims he had gone missing after approaching a bright light off the east coast. In 1992, The Grimsby Evening Telegraph printed an alleged transcript of his call back to base in which he reportedly said he had seen something "like a large soccer ball made of glass. It's like bobbing up and down and going from side to side slowly."

The MoD insisted in its reports that there had been nothing sinister and Captain Shaffner had misjudged how low he was flying and had then failed to operate his ejector seat properly.

The MoD said it recorded each sighting to help ministers answer questions from MPs in Parliament – but they were always kept on paper. Notes from March 1988 show that the plan for a full computerised database was spiked. A memo sent to staff said it "contravened" statements from ministers saying UFOs did not pose a threat to the UK and resources would not be diverted to investigate incidents.

"I also understand that there was some concern about public reaction if knowledge of the work being undertaken emerged in the media," an official wrote. He said it now seemed "all work must stop", but that UFO incidents would continue to be logged "as and when" they took place.